This invention relates generally to pressure sensors and more particularly to pressure responsive variable parallel plate capacitive transducers. Such transducers are shown and described, for example, in U.S. Pat. No. 4,716,492, assigned to the assignee of the present invention. A capacitive transducer is shown in the patent having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation on a ceramic base, with metal coatings deposited on respective opposing surfaces of the diaphragm and the base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A cup-shaped connector body of electrical insulating material is fitted over the electrical circuit and is secured to the transducer by a housing sleeve which has a port for exposing the transducer diaphragm to an applied fluid pressure. The diaphragm is movable in response to variations in pressure applied to the diaphragm to vary the capacitance of the capacitor and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
In copending, coassigned patent application Ser. No. 07/972,680 filed Nov. 6, 1992, a capacitive pressure transducer is shown and described in which metal capacitor plates are disposed on opposite sides of two surfaces defining a cavity or gap formed closely adjacent an outer surface of a monolithic ceramic body. The ceramic comprises conventional material, such as alumina, provided in powdered form coated with an organic binder pressed into first and second portions, i.e., a diaphragm and a base having a recess formed in an outer face surface, by pressing the powder in a die. Metallized coatings are deposited as by screen printing a thick film paste on one surface of the diaphragm portion and on the recessed outer face surface of the base portion. The vehicle used in the thick film paste is then removed as by heating. A spacer of organic material may be placed in the recess to ensure that the cavity gap is maintained during a following pressing step in which the two portions are pressed together to form a single unit. The unit is then heated to a first debinderizing temperature. After the organics, including the spacer means, are vaporized/decomposed and released through still open cells of the ceramic, the unit is placed in a high temperature oven and co-fired in a reducing atmosphere with the metal layers forming a conductive coating bonded to the ceramic and the ceramic being sintered together to form a monolithic, closed cell body. In a modified embodiment, low temperature ceramic materials are used for the ceramic which can be sintered at a temperature low enough to permit use of conventional printed circuit inks fired in an air atmosphere.
The use of consumable spacer material, as noted in the above referenced application, is effective in forming closely controlled gaps limited essentially along an x-y plane in a monolithic ceramic body; however, there is a need to provide structures having controlled gaps along the z direction as well. For example, in making a monolithic differential pressure sensor in which two flexible diaphragms are placed over recesses formed on opposite sides of a base with a force transfer pin extending between the diaphragms and disposed in an opening in the base along the z axis, a gap must be maintained between the transfer pin and the sidewall forming the opening to permit unfettered motion of the pin under an applied force. However, the step of pressing the green ceramic components together in assembling the unit tends to close this gap thereby interfering with such movement.